on cooling, in thin crystalline plates, possessing a very brilliant lustre ; these melt at 205-206°. The yield is nearly theoretical.

The following analytical data were afforded by a well-crystallised specimen :

0.2776 gram gave 23 c.c. of nitrogen at 14° and 756·6 mm. 0.2384 gram gave 0.636 gram CO2 and 0·1520 gram H2O.

Succindibenzylamide is sparingly soluble in hot or cold ether, sparingly soluble in chloroform, almost insoluble in carbon bisulphide, slightly soluble in boiling benzene, insoluble in the cold, and insoluble in water.

It is not decomposed by boiling with aqueous soda.

Unsuccessful attempts were made to obtain a mercurial or silver derivative of this diamide; freshly precipitated mercuric oxide is not acted upon by the boiling alcoholic solution of the amide.

CO NHÍCH,CH

6. Succinmonobenzylamide, C2H.CO.NH,

This amide is formed by the action of ammonia on succinbenzylimide, thus::

.CO.

CH<co>N•C,H, + NH

CONH C,H,

= C,H,<CONH

For this purpose the succinbenzylimide is heated at 100° for 6-8 hours in a sealed tube with an excess of a strong solution of ammonia in alcohol.

Under these conditions, the yield of the amide never exceeds 30-34 per cent. of theory, and even when the temperature is carried to 200° and maintained for several hours, the yield is not materially increased.

In order to separate the amide, the contents of the tube are evaporated to dryness on the water-bath, and the solid residue is digested with chloroform, which readily dissolves the unaltered imide, leaving behind the amide; the latter is purified by dissolution in boiling alcohol, from which it separates on cooling in glistening microscopic prisms which melt at 189°.

A specimen, after drying at 100°, gave the following result:

0-1911 gram gave 228 c.c. of nitrogen at 20° and 769 mm.

Succinmonobenzylamide, in its behaviour towards solvents, resembles the diamide, from which it is readily distinguished by the ease with which it gives off ammonia on boiling with fixed alkali. On heating, it first melts, and at a higher temperature parts with ammonia, regenerating the imide.

The following comparatively easy decomposition of normal benzylammonium succinate is worthy of note.

A small quantity of the syrupy mother-liquor from some dibenzylammonium succinate was evaporated in a glass dish on the water-bath as far as possible, dried for a couple of hours in a wateroven, and placed in a desiccator over oil of vitriol, where it remained untouched for about seven weeks. On attempting to redissolve the product in water, it was found that a portion representing about 20 per cent. of the whole refused to dissolve. This was collected, washed, and crystallised from hot alcohol, in which it was easily soluble. The crystals which separated were easily seen to be a mixture of two compounds, and the melting point, 196°, did not agree with any compound described in this paper. The crystals were directly digested with chloroform, and a rather sharp separation was thus effected, the products* so obtained gave the following results::

I. Product insoluble in chloroform; m. p. 205—206°.

0.1864 gram gave 16 c.c. of nitrogen at 180° C. and 762 mm. N 9.88 per cent.

II. Product soluble in chloroform and left as residue on evaporation; m. p. 102-103°.

0-1956 gram gave 134 c.c. nitrogen at 17° C. and 760.5 mm., or N = 7.91 per cent.

The quantity of material at my disposal was too small to attempt a further purification.

VOL. LV.

2 Y

Considering that the compounds were not quite pure, the analytical results and melting points leave no doubt as to their identity. A few hours' heating in the water-oven and exposure for several weeks over oil of vitriol were, therefore, sufficient to cause a partial loss of the elements of water and benzylamine from anhydrous benzylammonium succinate, and the production of a mixture of the amide and imide.

University Laboratory,

Trinity College, Dublin.

LXIII.-The Periodic Law of the Chemical Elements.
By Professor MENDELÉEFF.

(FARADAY LECTURE delivered before the Fellows of the Chemical Society in the Theatre of the Royal Institution, on Tuesday, June 4th, 1889.)

THE high honour bestowed by the Chemical Society in inviting me to pay a tribute to the world-famed name of Faraday by delivering this lecture has induced me to take for its subject the Periodic Law of the Elements-this being a generalisation in chemistry which has of late attracted much attention.

While science is pursuing a steady onward movement, it is convenient from time to time to cast a glance back on the route already traversed, and especially to consider the new conceptions which aim at discovering the general meaning of the stock of facts accumulated from day to day in our laboratories. Owing to the possession of laboratories, modern science now bears a new character, quite unknown not only to antiquity but even to the preceding century. Bacon's and Descartes' idea of submitting the mechanism of science simultaneously to experiment and reasoning has been fully realised in the case of chemistry, it having become not only possible but always customary to experiment. Under the all-penetrating control of experiment, a new theory, even if crude, is quickly strengthened, provided it be founded on a sufficient basis; the asperities are removed, it is amended by degrees, and soon loses the phantom light of a shadowy form or of one founded on mere prejudice; it is able to lead to logical conclusions and to submit to experimental proof. Willingly or not, in science we all must submit not

to what seems to us attractive from one point of view or from another, but to what represents an agreement between theory and experiment; in other words, to demonstrated generalisation and to the approved experiment. Is it long since many refused to accept the generalisations involved in the law of Avogadro and Ampère, so widely extended by Gerhardt? We still may hear the voices of its opponents; they enjoy perfect freedom, but vainly will their voices rise so long. as they do not use the language of demonstrated facts. The striking observations with the spectroscope which have permitted us to analyse the chemical constitution of distant worlds, seemed, at first, applicable to the task of determining the nature of the atoms themselves; but the working out of the idea in the laboratory soon demonstrated that the characters of spectra are determined—not directly by the atoms, but by the molecules into which the atoms are packed; and so it became evident that muore verified facts must be collected before it will be possible to formulate new generalisations capable of taking their place beside those ordinary ones based upon the conception of simple bodies and atoms. But as the shade of the leaves and roots of living plants, together with the relics of a decayed vegetation, favour the growth of the seedling and serve to promote its luxurious development, in like manner sound generalisations— together with the relics of those which have proved to be untenable -promote scientific productivity, and ensure the luxurious growth of science under the influence of rays emanating from the centres of

scientific energy. Such centres are scientific associations and societies. Before one of the oldest and most powerful of these I am about to take the liberty of passing in review the 20 years' life of a generalisation which is known under the name of the Periodic Law. It was in March, 1869, that I ventured to lay before the then youthful Russian Chemical Society the ideas upon the same subject, which I had expressed in my just written "Principles of Chemistry." Without entering into details, I will give the conclusions I then arrived at, in the very words I used :—

"1. The elements, if arranged according to their atomic weights, exhibit an evident periodicity of properties.

"2. Elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (e.g., platinum, iridium, osmium) or which increase regularly (e.g., potassium, rubidium, cæsium).

"3. The arrangement of the elements, or of groups of elements in the order of their atomic weights corresponds to their so-called valencies as well as, to some extent, to their distinctive chemical properties-as is apparent among other series in that of lithium, beryllium, barium, carbon, nitrogen, oxygen and iron.

"4. The elements which are the most widely diffused have small atomic weights.

"5. The magnitude of the atomic weight determines the character of the element just as the magnitude of the molecule determines the character of a compound body.

"6. We must expect the discovery of many yet unknown elements, for example, elements analogous to aluminium and silicon, whose atomic weight would be between 65 and 75.

"7. The atomic weight of an element may sometimes be amended by a knowledge of those of the contiguous elements. Thus, the atomic weight of tellurium must lie between 123 and 126, and cannot be 128.

"8. Certain characteristic properties of the elements can be foretold from their atomic weights.

"The aim of this communication will be fully attained if I succeed in drawing the attention of investigators to those relations which exist between the atomic weights of dissimilar elements, which, as far as I know, have hitherto been almost completely neglected. I believe that the solution of some of the most important problems of our science lies in researches of this kind."

To-day, 20 years after the above conclusions were formulated, they may still be considered as expressing the essence of the now wellknown periodic law.

Reverting to the epoch terminating with the sixties, it is proper to indicate three series of data without the knowledge of which the periodic law could not have been discovered, and which rendered its appearance natural and intelligible.

In the first place, it was at that time that the numerical value of atomic weights became definitely known. Ten years earlier such knowledge did not exist, as may be gathered from the fact that in 1860 chemists from all parts of the world met at Karlsruhe in order to come to some agreement, if not with respect to views relating to atoms, at any rate as regards their definite representation. Many of those present probably remember how vain were the hopes of coming to an understanding, and how much ground was gained at that Congress by the followers of the unitary theory so brilliantly represented by Cannizzaro. I vividly remember the impression produced by his speeches, which admitted of no compromise, and seemed to advocate truth itself, based on the conceptions of Avogadro, Gerhardt and Regnault, which at that time were far from being generally recognised. And though no understanding could be arrived at, yet the objects of the meeting were attained, for the ideas of Cannizzaro proved, after a few years, to be the only ones which could stand criticism, and which represented an atom as-" the